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r T MEADE M THE AmERICAN Journal of Science, Vol-. XXXIV, August, 1912] 

GenCol 1 


The COMPOSITION, STRUCTURE, and HARD¬ 
NESS of SOME PERUVIAN BRONZE AXES. 


By H. W. Foote and W. H. Buell. 


(Contributions from the Yale Peruvian Expedition, 
Yale University, New Haven, Conn., U* S. A.) 


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128 


Foote and Buell—Peruvian Bronze Axes. 


Art. XII.— The Composition, Structure, and Hardness of 

Some Peruvian Bronze Axes; by H. W. Foote and W. H. 

Buell. 

During the summer of 1911, Prof. Hiram Bingham, while 
with the Yale Peruvian expedition in eastern Peru, obtained 
three bronze axes which appear to belong to the Inca period. 
These axes were turned over to the authors and form the basis 
of the present investigation. 

In the photograph of the axes, fig. 1, we have numbered 
the specimens and given the dimensions. Nos. 1 and 3 were 


Fig. 1. 



1 . 2 . 3 . 

found on the site of an old Inca settlement near the Pampaconas 
Eiver in latitude 12° 30' S. and longitude 73°. This river 
has not yet been mapped. It is a western branch of the 
Urubamba Eiver, flowing nearly parallel and emptying into 
it at some point below Icharate. The settlement where these 
axes were found had been entirely deserted and all the original 
clearings were covered with forest. A few years ago, a small 
tract was cleared and it was in this new clearing that the axes 
were found. Axe No. 2 was found in the valley of the 
Urubamba Eiver near Eosalina. 

In our investigation, we have determined, first, the chemical 
composition of the axes; second, their micrographic structure, 
and third, their hardness. By comparing the structure of one 
of the axes with that of a new alloy of the same composition, 
we have been able to draw conclusions as to the methods used 
originally in making the axes. 

The following results were obtained on analysis : 


Gift 

The University 
27 1913 













Foote and Buell—Peruvian 

Bronze Axes. 


No. 1 

* No. 2 

No. 3 

Tin_ 

_ 12*03 

5’58 

3-36 

Copper . 

. 88-06 

93-94 

96-44 

Iron ... . 

_ 008 

none 

trace 

Silver. 

. none 

0-65 

none 

Sulphur. 

. .... 0-35 

0-08 

0-23 

Lead.. 

. none 

trace 

none 


100-52 

100-25 

100-03 


The results show that the alloys are all bronzes which vary 
considerably in their percentage of tin and contain a small 
amount of sulphur. No. 2 also contains silver in small amount. 

The alloys of copper and tin have been most thoroughly inves¬ 
tigated by Hey cock and Neville* and by Shepherd and Brough, f 
Their results show that alloys containing not more than approx¬ 
imately 10 per cent of tin consist entirely of a solid solution of 
tin in copper, stable at all temperatures below the melting 
point, and made up of so-called a-crystals. Alloys containing 
between 10 and 22 per cent of tin deposit on solidifying a 
mixture of a- and /3-crystals. The /3-crystals consist also of a 
solid solution of the two metals but contain more tin than the 
a modification. They are stable only above 480° C. and if 
cooled slowly below this transition temperature, 8-crystals are 
formed. These are extremely brittle and an alloy containing 
them cannot be successfully worked either hot or cold. If, 
however, the alloy is quenched rapidly from a temperature 
above 480° C., a mixture of a- and /3-crystals results which is 
characterized by great strength and tenacity. This alloy can 
be forged. It is evident, therefore, that the properties of 
an alloy whose composition is between the limits mentioned 
above, depend very largely on its previous heat treatment, 
since the properties of /3- and 8-crystals are different. 

Referring to the analyses of the bronzes, No. 1 appeared to 
be the most interesting from a metallurgical standpoint, con¬ 
taining, as it does, 12 per cent of tin, or enough to yield either 
/3- or 8-crystals. For comparison, a bar of metal was cast of 
nearly the same composition as this axe. It contained : 

Tin ..12 per cent 

Copper...88 

From this bar an axe was forged which we shall call the “ new 
axe.” In forging the new axe, we endeavored first to work it 
hot without paying much attention to the temperature, but 
found this could only be done above 500° C. After heating 
followed by slow cooling, the alloy was still extremely brittle. 

*Phil. Trans., ccii A, 1, 1903. f Jour. Pkys. Chem., x, 630, 1906. 













130 


Foote arid Buell—Peruvian Bronze Axes. 


Under a hammer, it broke in pieces. By heating above 500° 
C. and quenching, however, to retain the /3-crystals, we were 
able to forge the axe cold without any material difficulty, but 
it was found necessary to anneal at intervals during the forg¬ 
ing, heating each time above 500° C. and quenching. 

Photographs were made showing the microstructure of the 
original casting from which the new axe was made, before the 
alloy had been worked or annealed. Fig. 2 shows this struc¬ 
ture, which is characteristic of alloys of this type, made up of 
fern-like or kidney-shaped ‘casting crystals.’ Figs. 3 and 4 show 
the structure of the new axe in its finished condition after 
annealing and forging as described above. Fig. 3 was taken near 
the center of the axe and shows the crystals with no distortion. 
Fig. 4 was taken near the edge and shows slight distortion of 
the crystals due to forging. From a comparison of figs. 2, 3, 
and 4, it is evident that the structure has been entirely changed 
by annealing and forging. 

The original axe, No. 1, shows from its shape, and from the 
Fig. 2. Fig. 3. 



Fig. 2. Showing casting structure of new axe. 

Fig. 3. Showing structure on center of finished axe. Made from metal 
shown in fig. 2. 


marks on it, that it has been forged. The original shape of 
the casting cannot be told, but there can be no doubt that the 
shape has been materially changed by the forging. Figs. 5 
and 6 show the micro-structure of this axe. It is similar in 
type to that shown in figs. 3 and 4 and entirely different from 
that in fig. 2. The size of the crystals is different. That, 
however, is controlled by the temperature before quenching. 
The size of crystals produced by annealing is usually a 


Poote and Buell—Peruvian Bronze Axes. 


131 


Fig. 4. Fig. 5. 



Fig. 4. Showing structure on edge of new axe. Made from metal shown 
in fig. 2. Crystals somewhat distorted due to hammering. 

Fig. 5. Showing crystals near edge on axe No. 1. The structure of 
the metal was poor and it was impossible to get a very good etching. 

function of the temperature and the size can be increased by 
raising the temperature. The time of heating is not impor¬ 
tant except at low temperatures or when the heating lias been 
continued for a long time, for example, for several days. 


Fig. 6. Fig. 7. 



Fig. 0. Showing crystals in center of axe No. 1. The structure of 
the metal was poor and it was impossible to get a very good etching. 

Fig. 7. Showing fine crystals, which occur only at one point of edge on 
axe No. 3. 

Taking into account the facts of micro-structure and that 
the axe has beeil forged, it is fair to infer that after casting 
the original alloy, it was heated to a temperature considerably 
above 500° and either forged hot or quenched suddenly and 
forged cold. This required a very considerable degree of skill 
on the part of the original makers. 




132 


Foote and Buell—Peruvian Bronze Axes. 


Fig. 8. 



Fig. 8. Showing crystals on axe No. 3, a short distance from edge on 
cross section. 

We were unable to obtain an etching of axe No. 2. Appar¬ 
ently the silver in it gave trouble by dissolving in all the 
etching solutions which we tried and reprecipitating on the 
surface, leaving a dark deposit which prevented the micro¬ 
structure from being observed. Figs. 7 and 8 show the micro¬ 
structure of the fragment of axe No. .3. Eoth views were 
taken on the broken section. The fine crystals occurred close 
to the edge at the extreme left as shown in fig. 1 and the 
coarse crystals occurred nearer the center. The axe was 
therefore very unevenly heated, either in the original anneal¬ 
ing or at some time afterward. 

None of the old axes are very hard. The following table 
gives the results of the scleroscope tests : 

New axe No. 1 No. 2 No. 3 


Edge... 36 26 18 18 

Center_ 34 24 20 


The hardness was taken with the Shore scleroscope and 
it varies somewhat with the percentage of tin as would be 
expected, but shows nothing at all unusual. In fact, the hard¬ 
ness of the old axes is considerably less than that of the new 
axe, which is probably due to coarser structure as well as to 
composition. The values given in the table are expressed in 
empirical units and merely give comparative values among 
themselves. For the sake of comparing the hardness of the 
axes with that of another metal, it may be well to add that a 
piece of cast brass has a hardness on the same scale of 11 or 12 
units. 

Chemical Laboratories of the Sheffield Scientific School 
and of the Winchester Repeating Arms Co. 

New Haven, Conn., May, 1912. 





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